Re: [PATCH 1/5] dt/bindings: Add binding for the DA8xx MUSB driver

[Petr Kulhavy <petr-Qh/3xLP0EvwAvxtiuMwx3w@public.gmane.org>]

Hello Rob,

On 16.02.2016 21:05, Rob Herring wrote:
> On Fri, Feb 12, 2016 at 11:24 AM, Petr Kulhavy <petr-Qh/3xLP0EvwAvxtiuMwx3w@public.gmane.org> wrote:
>> I understand your point that the description should be generic.
>> However the USB 2.0 specification does not define any relation between the
>> bMaxPower provided by the device and the actual power control.
>> As far as I understand the value just serves the purpose to raise a flag to
>> the user if the attached device would draw too much power, and not to enable
>> it at all.
> That won't really work given devices lie. My bus powered USB disk
> enclosure reports something like 10mA. That's pretty good considering
> I have a 5W drive in it.
Yes, that's exactly what's happening. The USB 2.0 is in that matter 
naive and assumes that the devices don't lie :-)
But at the same time gives them the possibility to lie by not binding 
this value to the physical current.
>> For instance on my development board the real maximum current is limited
>> only by the mains power-supply used.
>> So it cannot even be modelled in the DT as a regulator because the
>> power-supply is not part of the HW and
>> everybody can take a different one.
> Not part of which h/w? Different for everyone is exactly why Vbus
> supply should be described in DT.
I mean the regulator is in the AC power supply, which is standalone and 
can be changed. Therefore not described in the DT.
The DT describes only the board.

>> So defining a regulator just to store this arbitrary USB 2.0 value is a bit
>> overkill for me.
> If it is just arbitrary, then put it into the driver. I would do 500mA
> and be done with it. I'd guess there is nothing real behind the
> current default of 250mA.
Still there is some sense behind. The power_budget is used to possibly 
print a warning message and/or
disable a self powered device that announces too much power. So I would 
leave it configurable.

What would be your proposal then? Something like this?

     usb_reg: fixedregulator@0 {
         compatible = "regulator-fixed";
         regulator-name = "USB Vbus";
         regulator-always-on;
         regulator-max-microamp = <500000>;    /* 500mA */
     };

or a real regulator if there is one. And then reference it as:

     usb20: ... {
         ...
         vbus-supply = <&usb_reg>;
     }

As I said for me it's an overkill, knowing that this monster needs to be 
set-up and reside in memory just to write the power_budget variable for 
devices which anyway may lie.
But if you prefer that and Sergei and the guys from USB agree to it I 
don't really mind.--

Regards
Petr
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